This research is directed toward the development of a new technique for the application of the nuclear magnetic resonance-lanthanide shift reagent method to the determination of molecular conformations in solution. The method is being used to study the conformations of a number of substituted diphenyl ethers which are structural analogs of the thyroid-active thyronines. A complex is formed in solution between a lanthanide shift reagent and a conformationally rigid, bicyclic organic substrate, which is connected to a conformationally flexible substituent (e.g., a diphenyl ether). The nmr-lanthanide induced shift data for the bicyclic substrate, together with the three dimensional structure of the substrate from X-ray crystallographic data, permits the determination of the lanthanide-substrate structure in solution. This data and the shift data for the flexible substituent permit the determination of its conformational properties. The conformational properties of several iodinated diphenylether-bicyclic compounds are being evaluated.